1. Field of the Invention
The present invention relates to a turboexpander pump unit, and more particularly to a turboexpander pump unit for use in a liquefied gas supply installation suitable for use in storing, transporting, and supplying a cryogenic liquid fuel such as a liquefied natural gas (LNG) or the like.
2. Description of the Prior Art
FIG. 19 of the accompanying drawings shows the concept of a conventional liquefied gas supply installation in an LNG base. An LNG unloaded from a transport ship is stored in a partly underground tank 201. The LNG stored in the tank 201 can be lifted by a primary (first stage) pump 202 immersed in the stored LNG. A portion of the LNG lifted out of the tank 201 is gasified by an evaporator 203 and delivered as a fuel for a boiler or a gas turbine in the LNG base. The evaporator 203 introduces seawater or waste hot water from an inlet 203A and discharges it from an outlet 203B, during which time the LNG is gasified by a heat exchange in the evaporator 203. Most of the LNG lifted by the pump 202 is pressurized by a secondary (second stage) pump 204, and either supplied in a liquid state to another LNG base through a pipeline 205 or subsequently gasified with heat by a heat exchanger (not shown) and delivered under pressure as a gas for generating electric energy or a city gas to a region where it is to be consumed.
The pump for pressuring the ultra low temperature LNG is generally in the form of a multistage vertical centrifugal pump, and is of the submerged type in which a pump and a motor for driving the pump are entirely submerged in the LNG to eliminate the possibility of leakage from sealed shaft portions (for details, see "Operation and control of LNG devices" written by Aizawa and Kubota, TURBOMACHINES, vol. 17, No. 5, pages 8-13).
Recent years have seen growing demands for LNG as a clean energy source suitable for environmental protection, and increasing LNG service areas have required liquefied gas supply devices to have a larger capacity, a greater scale, and a more ability to handle a higher gas pressure. The secondary pump 204 which is a main pump for delivering the LNG under pressure is, therefore, required to handle a greater gas flow rate and a higher head, and to be driven by a larger horsepower. A motor for driving the pump 204 needs a high-voltage electric energy supply installation having a large power capacity ranging from several hundreds to several tens of thousands kW, and, as a result, also needs a large electric energy transmission and distribution installation for transmitting and distributing electric energy to the motor.
As the number of stages and the size of the pump increase, an installation space and a maintenance procedure required by the pump pose problems. It has been customary to transport the LNG through a long pipe to a remote electric power generating station to generate electric energy, and supply the generated electric energy from the electric power generating station through long electric cables to the LNG pressure-delivery pump in the LNG base where the supplied electric energy is supplied to energize the motor. Such an electric energy supply system is not preferable from the standpoint of energy saving efforts. Stated otherwise, the supply of electric energy to the LNG pressure-delivery pump in the LNG base has resulted in a transport loss caused by the delivery of the LNG in a gas or liquid state to the electric power generating station, an energy conversion loss caused in the electric power generating station, a transport loss caused by the electric cables, and an energy conversion loss caused by the rotation of the motor.
The submerged pump has a problem in that magnetic bearings are required to be used on the iron core of the rotor of the motor. Since magnetic iron plates are made of ferrite, they are brittle and have low tolerances for tensile or bending stresses at low temperatures. Therefore, the rotational speed of the motor cannot be increased due to limitations on centrifugal stresses. If the motor is of high output power, then the rotor thereof is required to be long enough to have low inherent values, which would make it difficult to get a suitable motor design available even with the above-mentioned rotational speeds.